functions.php

<?php

/**
 * Get Page Hierarchy
 *
 * returns array
 * -- pages (all pages in order)
 * -- prev (previous page)
 * -- next (next page)
 * 
 * @author Bill Erickson
 * @link http://www.billerickson.net/code/get-page-hierarchy/
 */
function be_get_page_hierarchy() {
	global $post;
	$loop = new WP_Query( array( 
		'post_type' => 'page',
		'post_parent' => $post->post_parent,
		'posts_per_page' => -1,
		'orderby' => 'menu_order',
		'order' => 'ASC',
		'fields' => 'ids',
	) );
	$output = array(
		'pages' => $loop->posts,
	);
	foreach( $loop->posts as $k => $v ) {
		if( $v == get_the_ID() ) {
			if( isset( $loop->posts[$k-1] ) )
				$output['prev'] = $loop->posts[$k-1];
			if( isset( $loop->posts[$k+1] ) )
				$output['next'] = $loop->posts[$k+1];
		}
	}
	return $output;
}